The present invention pertains to the fabrication of A-15 type multifilament composite superconductors (By xe2x80x9cA-15xe2x80x9d is meant the inter metallic compounds having xcex2W structure). These include Nb3Sn, and Nb3Al and are important because of their superior high field properties. Unfortunately, they are brittle compounds, difficult to make as fine filaments and as a result are very expensive. For these reasons, the ductile NbTi superconductors has dominated the commercial market even though its maximum magnetic field are limited to less than 8 Tesla. Significant improvements are needed in order to commercialize the A-15 conductors and to extend the useful magnetic fields to the 12 Tesla range on a cost effective basis. The present invention is also applicable to the production of the xe2x80x9cB1xe2x80x9d superconductors NbN and NbC.
Standard Industrial Practice
A detailed description of present day methods currently being used in the industry is described in an article entitled xe2x80x9cA-15 Superconductorsxe2x80x9d in the Metals Handbook, Tenth Edition, Volume 2 on pages 1060-1077, authored by David B. Smathers. Two processes are currently being used. One employs a bronze alloy as the matrix, the other a combination of pure copper and a pure Sn core. The first is known as the xe2x80x9cBronze Processxe2x80x9d and the latter the xe2x80x9cInternal Tin Processxe2x80x9d. The bronze matrix contains up to 13% Sn work hardens rapidly and requires frequent annealing steps. These anneals are avoided with the internal tin process. However, the cold drawing in the Internal Tin process can result in poor bonding, degraded filament quality and poor yields. After final drawing and twisting, both types of conductors are heated to approximately 700xc2x0 C. for 200 hours or more to form Nb3Sn. Magnets are made exclusively by the Wind and React method.
The current densities that are obtained are substantially below what is theoretically possible based on experimental short sample data. Contributing to this problem is the low reaction temp of up to 700xc2x0 C., requiring hundreds of hours of reaction heat treatment. As the Sn is depleted, the Sn gradient is reduced which further limits the reaction. Unreacted Nb in the Nb filament can be left and Kirkendall type voids are formed in the residual matrix resulting in a lowering of the current density and mechanical properties of the conductor.
Significant improvements are needed to improve the high field performance and to reduce the cost of these important A-15 conductors.
In the fabrication of Nb3Sn superconducting wire, a barrier, usually tantalum or a tantalum alloy is employed to prevent tin contamination of the stabilizing copper on the exterior of the wire during heat treatment. The process is described in the article by David B. Smathers. While the porous metal sheath described in the present invention is similar to the barrier used in Nb3Sn conductor fabrication, application of the technology in this invention is entirely unique.
Fiber Production
U.S. Pat. Nos. 5,034,857 and 5,869,196 by Wong, discloses a novel approach to the production of very fine valve metal filaments, preferable tantalum, for capacitor use. The benefits of fine filaments relative to fine powders are higher purity, lower cost, uniformity of cross section, and ease of dielectric infiltration, while still maintaining high surface area for anodization. The uniformity of cross section results in capacitors with high specific capacitance, lower ESR and ESL, and less sensitivity to forming voltage and sintering temperature as compared to fine powder compacts. Other patents involving valve metal filaments and fibers, their fabrication, or articles made therefrom include U.S. Pat. No. 3,277,564 (Webber), U.S. Pat. No. 3,379,000 (Webber), U.S. Pat. No. 3,394,213 (Roberts), U.S. Pat. No. 3,567,407 (Yoblin), U.S. Pat. No. 3,698,863 (Roberts) U.S. Pat. No. 3,742,369 (Douglass), U.S. Pat. No. 4,502,884 (Fife), U.S. Pat. No. 155,306,462 (Fife) and U.S. Pat. No. 5,245,514 (Fife).
The prior art relating to the fabrication of A-15 conductors can be obtained by reading both Smather""s article and xe2x80x9cFilamentary A-15 Superconductorsxe2x80x9d by Masaki Suenaga and Alan F. Clark, Plenum Press, N.Y. Cryogenic Material Series (published 1980). In this book, the article by C. H. Rosner, B. A. Zeitlin, R. CX. Schwall, M. S. Walker and G. M. Ozeryansky entitled xe2x80x9cReview of Superconducting Activities at IGC on A-15 Conductorsxe2x80x9d pages 67-79, specifically summarizes the earlier developments. Initially, powder metallurgy methods were employed followed soon by surface diffusion of liquid Sn of both Nb tapes and wires. Allen U.S. Pat. No. 3,218,693 patented a method where Sn coated Nb ribbons and wire were reacted to form Nb3Sn at temperatures between 800xc2x0 C. to 1000xc2x0 C. Similar products were also made by General Electric and later by IGC. The Nb wires in cable form, were Sn dipped, wound into a magnet and reacted; D. F. Martin et al U.S. Pat. No. 3,429,032. A subsequent article, by Scanlan and Fietz, xe2x80x9cMultiFilamentary Nb3Sn for Superconducting Generator applicationxe2x80x9d, IEEE Trans. MAG-11 page 287, March ""75, describes fabrication of a Nb3Sn cable employing electroplated Sn as the Sn source.
A new approach is necessary to improve the processing of Nb3Sn conductors. In the early 1960""s, pure Nb tapes, wires and cable were dipped in molten Sn baths which was then reacted at high temperatures to form Nb3Sn. Because Nb3Sn is brittle, a ductile substrate of unreacted Nb was left to permit handling and subsequent winding into magnets. However, the need for stable, fine filaments and twisted conductors soon made this method obsolete. Wong""s patent ""196 describes a process used to manufacture Ta capacitors where Ta multifilaments are made in a constraining sheath. This process describes the removal of the copper matrix after the final forming operation. The advantages of the external sheath is that final packaging of the filaments are unnecessary since the filaments are now constrained and supported by the outer sheath. Furthermore, the area inside the sheath is exactly determined as is the volume fraction of Ta.
A precursor wire, containing fine Nb filaments enclosed in a constraining sheath which can act as a supporting structure is produced. Upon the removal of the copper matrix, and employing a liquid Sn dipping process, Sn or a CuSn alloy is used to infiltrate and surround the Nb filaments. The significant advantage here is that the need for subsequent wire drawing is completely eliminated as is the co-processing and low yield difficulties of present day Nb3Sn conductors. The ability to easily increase the Sn concentration can result in substantial improvement in current density over present day conductors. The sheath, in the preferred embodiments, is made of Nb although Ta and stainless steel could also be used. The sheath is fabricated by methods as described in U.S. Pat. No. 5,869,196 by Wong. The billet is processed in the normal manner by extrusion and wire drawing to the final size. The copper matrix is then removed from this precursor wire and replaced with a Sn or CuSn alloy matrix. Final reaction heat treatments are then used to convert the Nb to Nb3Sn.